Cysteine (—S—S—) bridges are common structural motifs in naturally occurring cyclic peptides. In some cases, these disulfide bridges act as reactive functional groups. In many other cases however, the cysteine-bridge serves a skeletal, structural role, maintaining secondary and tertiary structure. Disulfide bonds in peptides and other compounds are highly reactive under broad-ranging conditions, and therefore useful peptides containing disulfide bonds that have a structural role are at risk of denaturation, resulting in loss of properties.